December 30, 2011

Climate and body composition

Am J Phys Anthropol DOI: 10.1002/ajpa.21591

Ecogeographical associations between climate and human body composition: Analyses based on anthropometry and skinfolds

Jonathan C.K. Wells et al.

Abstract

In the 19th century, two “ecogeographical rules” were proposed hypothesizing associations of climate with mammalian body size and proportions. Data on human body weight and relative leg length support these rules; however, it is unknown whether such associations are attributable to lean tissue (the heat-producing component) or fat (energy stores). Data on weight, height, and two skinfold thickness were obtained from the literature for 137 nonindustrialized populations, providing 145 male and 115 female individual samples. A variety of indices of adiposity and lean mass were analyzed. Preliminary analyses indicated secular increases in skinfolds in men but not women, and associations of age and height with lean mass in both sexes. Decreasing annual temperature was associated with increasing body mass index (BMI), and increasing triceps but not subscapular skinfold. After adjusting for skinfolds, decreasing temperature remained associated with increasing BMI. These results indicate that colder environments favor both greater peripheral energy stores, and greater lean mass. Contrasting results for triceps and subscapular skinfolds might be due to adaptive strategies either constraining central adiposity in cold environments to reduce cardiovascular risk, or favoring central adiposity in warmer environments to maintain energetic support of the immune system. Polynesian populations were analyzed separately and contradicted all of the climate trends, indicating support for the hypothesis that they are cold-adapted despite occupying a tropical region. It is unclear whether such associations emerge through natural selection or through trans-generational and life-course plasticity. These findings nevertheless aid understanding of the wide variability in human physique and adiposity.

13 comments:

"Polynesian populations were analyzed separately and contradicted all of the climate trends, indicating support for the hypothesis that they are cold-adapted despite occupying a tropical region."

It occures to me, then, that the opposite should trend true among Africans brought to the New World from the tropics and located in temperate climates. Am I missing something? The Polynesian body fat issue would seem to suggest something else related to climate/diet than climate/temperature.

Pascvaks says: "the opposite should tend true among Africans brought to the New World from the tropics and located in temperate climates."

Your example supports rather than undermines the generalization, since it illustrates the relative absence of a trend away from the ancestral type, just as we have with the Polynesians. In both cases the movement was too recent to have produced much of a change yet. So the Polynesians still retain some of the cold-adapted Mongoloid phenotype of their ancestors as the American Negroes retain some of heat-adapted Negroid phenotype of their ancestors. Only if the latter had changed greatly from their ancestors, and done so independently from the influences of Caucasoid admixture, would we have reason to wonder why Polynesians were not more heat-adapted. So in this case it would be premature to abandon the climate-based explanation for a dietary one.

Gregory76 said... "Only if the latter had changed greatly from their ancestors, and done so independently from the influences of Caucasoid admixture, would we have reason to wonder why Polynesians were not more heat-adapted. So in this case it would be premature to abandon the climate-based explanation for a dietary one."

Good point. But, inclination being what it is, there seems to be a hole in this paper regarding diet. New World African history seems an ideal area within which to assess the question. Perhaps not.

"the smaller, more lightly built were more likely to be eaten in emergency situations".

Possibly. Perhaps they would be less able to fight off larger people. Personally I would prefer to eat younger individuals whose flesh would be more tender. Same as young sheep or cattle.

"So the Polynesians still retain some of the cold-adapted Mongoloid phenotype"

That's unlikely to be the explanation. Polynesians are basically large-framed Malays rather than being Mongoloids. Malays (and Polynesians) display evidence of being a mixture between Mongoloids and Papuans.

" there seems to be a hole in this paper regarding diet. New World African history seems an ideal area within which to assess the question".

I'm sure that diet would have an effect, but I don't think that it explains the Polynesian differential. Even taking into consideration the fact that some groups practiced cannibalism.

Though the Mongoloid phenotype is somewhat diluted among the Malayans, they are still are mostly Mongoloid, and though the phenotype seems to be still more diluted among the Polynesians, the Mongoloid component is strong if not predominant among them. Insofar as the dilution of the Mongoloid phenotype among the Malayans is due to admixture with non-Mongoloid, it seems that Australoids may be involved, in addition to or instead of Papuans, and the dilution may be partly or wholly due to adaptation of Mongoloids to the warmer climate.In any case, since we know that the Polynesians are mostly descended from partially Mongoloid Malayans, and that a high BMI is associated with Mongoloids, the most parsimonious explanation for Polynesians having a higher BMI than would be selected for in their hot climate is that it is a result of their Mongoloid ancestry. So there still seems no need to abandon a climate-based explanation in favor of a diet-based one in this case.

"Though the Mongoloid phenotype is somewhat diluted among the Malayans"

'Somewhat diluted'? Considerably diluted more like. The expression Malay' applies equally to Indonesians and Filipinos, as well as to Polynesians.

"it seems that Australoids may be involved, in addition to or instead of Papuans"

Possibly. It depends on what people inhabited island SE Asia before the arrival of the Mongoloids. Quite likely a mixture of Australoids and Papuans.

"since we know that the Polynesians are mostly descended from partially Mongoloid Malayans, and that a high BMI is associated with Mongoloids, the most parsimonious explanation for Polynesians having a higher BMI than would be selected for in their hot climate is that it is a result of their Mongoloid ancestry".

Although the explanation you offer may be 'the most parsimonious explanation', that explanation is unlikely to be the correct one. The 'partially Mongoloid Malayans' that Polynesians are descended from do not possess the higher BMI. So some other explanation is required. Virtually everyone who has studied Polynesians offers the explanation I offered earlier.

"So there still seems no need to abandon a climate-based explanation in favor of a diet-based one in this case".

Obviously the authors of the paper under consideration here feel it necessary to exclude Polynesians because they fail to fit the expected result. But a climate-based explanation still fits. No diet-based explanation has ever been offered.

Saying that remnant Mongoloid traits are the causative factor of high BMI in Polynesians ignores that the Mongoloids geographically and genetically closest to Polynesians - ie the Vietnamese, the Malays, the southern Chinese, etc. - all have low BMI.

It could be that Polynesians retain traits that the Mongoloids around them have not - in which case drift or selection remains a question - or it could be that the Mongoloid profile is not as useful as some people like to think.

"ignores that the Mongoloids geographically and genetically closest to Polynesians - ie the Vietnamese, the Malays, the southern Chinese, etc. - all have low BMI".

Exactly.

"It could be that Polynesians retain traits that the Mongoloids around them have not"

That's a possibility, but unlikely. 'Polynesians' didn't strictly exist until 3000 years ago at most. So they are long removed from 'Mongoloids'. Some may find this Wiki link interesting:

http://en.wikipedia.org/wiki/Ethnic_Malays

Quote:

"Malays (Malay: Melayu Jawi: ملايو) are an ethnic group of Austronesian people predominantly inhabiting the Malay Peninsula, including the southernmost parts of Thailand, the east coast of Sumatra, the coast of Borneo, and the smaller islands which lie between these locations. The Malay ethnic group is distinct from the concept of a Malay race, which encompasses a wider group of people, including most of Indonesia, Philippines, and Oceania".

This 'Malay race' shares the Austronesian language with Polynesians. And:

"If one follow the original migration of a certain group of southern Chinese of 6,000 years ago, some moved to Taiwan (today's Taiwanese aborigines are their descendents), then to the Philippines and later to Borneo (roughly 4,500 years ago) (today's Dayak and other groups). These ancient people also split with some heading to Sulawesi and others progressing into Java, and Sumatra. The final migration was to the Malayan Peninsula roughly 3,000 years ago. A sub-group from Borneo moved to Champa in Vietnam roughly 4,500 years ago. Interestingly, the Champa group eventually moved to present day Kelantan in Malaysia. There are also traces of the Dong Song and HoaBinh migration from Vietnam and Cambodia. There was also the Southern Thai migration, from what we know as Pattani today. All these groups share DNA and linguistic origins traceable to the island that is today Taiwan, and the ancestors of these ancient people are traceable to southern China".

So the Polynesians come from Southern China, but from a period before South China had become as completely Mongoloid as it is today.

I am still not sure that the body type of the original Polynesians was more robust than that of other southern Mongoloids, in part because of the BMI figures. The article presents figures by region rather than individual ethnic groups. And the articles it cites on these groups are not always relevant to the problem, while studies of BMI seem to be mainly concerned with obesity. Well members of any group, with any natural body type, can make themselves fat (assuming enough food is available). Because of that fact, and the fact that some admixture with Westerners has taken place, I suspect that the original phenotype of the Polynesians (and the eastern Micronesians) was less robust—though still more robust than that of Australoids and Papuans, as a result of their Mongoloid ancestry.However, if it should turn out the original Polynesians (and eastern Micronesians) were more robust than that of other southern Mongoloids, I would try to explain it by an admixture of Melanesian blood (which we already have reason to suppose on other grounds, including linguistics). Now we have the question of where the robustness of the Melanesians comes from. Their closest relatives, the Papuans, are less robust on the average, though there may be large subgroups of them are significantly more robust than the average Papuan. I suspect that the trihybrid hypothesis of Birdsell may be again be useful here and that the robustness may come from the Caucasoid component (“Murrayians”).

"The article presents figures by region rather than individual ethnic groups".

Quite. And the Polynesians are confined to a particular region, but they don't fit the pattern.

"I am still not sure that the body type of the original Polynesians was more robust than that of other southern Mongoloids"

That is probably correct. The Polynesians presumably were originally 'southern Mongoloids' and looked much like Malays or Filipinos. They developed their robustness as they moved over open expanses of ocean.

"if it should turn out the original Polynesians (and eastern Micronesians) were more robust than that of other southern Mongoloids, I would try to explain it by an admixture of Melanesian blood (which we already have reason to suppose on other grounds, including linguistics)".

Melanesians basically fit the standard climate-based hypothesis. They are not near as 'robust' as are Polynesians. However they may be more robust than Papuans, but this would be again because of long-distance travel over open sea at night. However nowhere near such distances as the Polynesians were required to do.

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